Generally in sheet metal working technology, for example in sheet punching, certain components are always needed. These include tools, punches or drifts, a releasing plate or a releaser or stripper, and a cushion. In sheet punching, the machining is conducted in a way that a punch is used to make a hole in a sheet against the cushion, and the punch is drawn out of the hole either by a spring force or by another force, wherein the releasing plate prevents the punched sheet from rising with the punch, the plate remaining in its place. In sheet punching, there are two possibilities to conduct the machining: first of all in a way that the releasing plate is in contact with the sheet to be punched during each punching stroke; or the second alternative being that there is a certain air gap between the material to be punched and the releasing plate. Each method has its own advantages, i.e., when there is a contact with the plate at the punching stage, the plate will not vibrate during machining. On the other hand, when punching aluminium or materials which are soft or have a sensitive surface, the releasing force may become too great and leave scratches on the surface of the material. Because of this, it is preferred that both alternatives be available in a sheet metal work center. Thus the releaser plate could be kept in place during the punching strokes to achieve a permanent air gap, or the releaser plate could touch the material with every punching stroke.
Known sheet metal work centres use a single-tool system having a buffer, to which the drift is mechanically fixed, wherein it can be exchanged, if necessary, either manually, by a robot or with a manipulator, and a releaser plate, wherein both are controlled with a separate pressurized medium cylinder arrangement. This kind of a solution involves, however, the problem that two separate shaft systems are required for controlling the pressurized medium, and these must be synchronized with a numerical control (NC) unit. This tends to delay the working process. The single-tool system is relatively fast in the so-called passive mode, wherein the releaser, usually a releaser plate, is stationary at the distance of an air gap from the sheet to be worked during the punching stroke. Thus the buffer, with the drift, can conduct punching strokes even at very short intervals. The problem is present particularly when the so-called active mode is used, wherein the releaser plate is placed in contact with the surface of the sheet to be worked. Thus a need for so-called serial control arises, wherein the implementation of the NC control unit requires first the information that the releaser plate is in contact with the surface of the sheet to be worked before the buffer can be given a punching command. This fact will cause a delay of about one third in the working process when moving from the passive mode to the active mode.